The invention relates to a method and mechanism for the calcination of fine grained pulverulent material, particularly which is comminuted raw material for the making of cement, wherein the material is thermally pretreated by hot gases in a preheating and precalcination operation before delivery to a sintering kiln and delivery of the clinker from the kiln to a cooler.
It has been known in the manufacture of cement to provide an additional calcination device in the gas conduit between a sintering furnace, a preheating heat exchanger system and mechanism for the deacidification of the comminuted raw material for cement. With this deacidification, the rotary kiln completes the heat treating operation which is principally that of clinker formation, that is the calcination of the material to completion. By deacidification in a precalcination device, the rotary kiln has thermal relief so that increased efficiency and an increase in output of the entire calcination installation are possible, and the dimensions of the structure and particularly of the rotary kiln can be reduced.
With newly developed manufacturing installations, very large output yields on the order of 3,000 tons per day of clinker or even more are possible with the arrangement of precalcination structures between the rotary sintering kiln and the heat exchanger system. In these constructions, appreciable difficulties are encountered in carrying out in an optimal manner the additional precalcination operations in the heat exchanger, and difficulties are encountered in controlling the load conditions to attain the desired calcination of the charging material without over-calcining in the precalcining process. The deacidification of a particle of lime is dependent substantially on three variables, the deacidification temperature, the duration time of the particles in the hot gas treatment stream and the grain size of the particle.
The grain size and grain distribution of the comminuted raw material are subject to technical limits. The production of a single grain comminuted material is at the present time not possible. An improvement of the deacidification conditions of the comminuted raw material by means of a change of this variable has previously led to no improved results.
The duration of time of the particles in the calcination step of a suspension gas heat exchanger system may be stretched out to a period of no longer than several seconds. The coarse grain fraction of the comminuted material, at temperatures of 850.degree. to 900.degree. C. requires deacidification periods which are appreciably higher. It has been suggested for the prolongation of the duration time to undertake a recirculation of a portion of the fine comminuted raw material to be deacidified. There is known from the German Laid-Open Specification No. 22 47 172, an installation for the manufacture of cement with a suspension gas heat exchanger including several cyclone steps arranged superimposed, with a rotary kiln, as well as an additional calcination chamber, in which in the material discharge conduit of the hottest cyclone, a correction or setting member is arranged through which a partial quantity of the material already calcined to a substantial extent is supplied to the additional calcination chamber. This is heated with combustion gases and additional fuel in which the fine grained material is to be calcined. In addition to the increase in cost for apparatus which is unfavorable to this installation, a uniformly calcined comminuted material before the sintering in the rotary kiln cannot be attained. The distribution of the comminuted raw material in accordance with the partial quantities required results in a part of the already deacidified material passing several times through the calcination chamber, and with a high degree of probability, a portion is over-calcined while another portion is only incompletely calcined and so arrives at the rotary kiln. Another problem is that the molten phase is reached and adhesions are caused on the gas and material guiding installation parts which are heated to the highest temperature, and these adhesions substantially impair the furnace operation. Also, deacidification heat must be applied in the furnace itself, and its production costs are unfavorably affected.
In accordance with another cement production method as shown in German Patent No. 23 44 583, there is arranged a calcination shaft between a rotary kiln and a heat exchanger of the suspension type. This shaft includes a lower combustion chamber with a separate fuel feed, a narrowed shaft part and an upper combustion chamber, whereby an additional fuel feed is arranged in the upper chamber above the shaft constriction. In the central part of the shaft, the arrangement contemplates a vigorously swirling layer of material so that a longer period of dwell of the material required for the calcination reaction is to be attained. It is not possible, however, to adjust the individual streams of gas in the treatment shaft to one another so that they can be referred to the particular granulation compositions of the material and cannot be arranged to attain the period of dwell required in each case for calcination. This results in partially over-burned and partially under-burned material.
In the German Laid-Open Specification No. 23 61 427, there is provided a specially constructed calcination chamber for the calcination of comminuted raw material for cement between a suspension gas heat exchanger and a rotary kiln. The calcination chamber provides for gas material suspension during formation of turbulent streams and is to be held for an extended period of dwell in the hot gas combustion gases in order to obtain a complete calcination. Such construction has the disadvantage of additional costs for the calcination chamber and the periods of dwell of the material cannot be adjusted, so the danger exists of over calcination of the material.
In order to influence the deacidification temperature, in another known method of German Laid-Open Specification No. 23 24 519, a comminuted raw material is to be calcined in a furnace exhaust conduit leading from the rotary kiln to the hottest zone of the preheater by means of additionally supplied fuel. This fuel is added to the material before entry into the exhaust gas conduit in finely divided form. This has the disadvantage that particularly with large cement manufacturing installations with high output that, a structure of appreciable dimensions must be provided. With such structure, a uniform heat transfer of fuel to the particles of comminuted raw material cannot be attained on account of the different concentration of the comminuted raw material and the exhaust gases. Additionally, the operation must be conducted with an appreciable excess of fuel because of the requirements for stabilization of the calcination so that maximal temperatures predetermined by the material can be uncontrollably exceeded. The fusible pulverulent material passes through the calcination and commences grain coarsening and commences formation of the molten phase. In this undesirable condition, the flow of the calcined material out of the hottest step of the preheater into the rotary kiln can create difficulty.
It is an object of the present invention to provide solutions to the foregoing and other disadvantages of the prior art and to provide a method and mechanism in which materials having a size from fine grain to dust can be calcined as uniformly as possble and be completely calcined in the precalcination zone of a heat exchanger system, even with very large output yields.
It is a further object of the invention to provide additional combustion zones for the precalcination operation which can be included in new systems or may be readily adapted in the area of presently existing heat exchanger systems so that constructions may be new or revised without requiring extensive capital and installation parts previously used or previously designed can be used in their present construction and size.
In accordance with the principles of the present invention, the inventive heat treatment step includes at least two reaction zones. The material to be treated is preheated and is passed through first and second reaction zones and the hot gases are separated from the material after each of the reaction zones and before the material is passed onto the subsequent location where it is treated. In this manner, a long period of dwell of the material in the temperature range required for the calcination is attained. In the particular reaction section, only as much fuel is added to the material as may be optimally mixed with the material reliably in the section for optimum calcination. During the separation of the gas and material, an intensive mixing of the material and an equalization of the material temperature takes place. This material then upon entry into the next following reaction section has a unitary temperature level and a uniform degree of calcination. The following reaction zone, therefore, requires only as much residual quantity of fuel to be supplied to it and converted to heat as required for the complete calcination, that is, for the attainment of a predetermined degree of calcination. In this manner, with optimal utilization of fuel, optimum and determinable prolongation of the time required for the calcination reaction is attained. Thereby, an over-calcination of the material particles is prevented. Also prevented are other disturbances such as caking in the heat exchanger system. This attains an improvement in the overall effectiveness and efficiency of the furnace as a unit.
In a further feature of the invention, fuels of different heat contents are supplied to the separate reaction zones. At least one of these will be a high grade fuel. This attains a good adaptation to the different fuel qualities. It has been found that primary energy is not always available with the required high heating values, and further that different factors require utilization of fuel of lesser quality.
A further feature of the invention provides for the use of a fuel containing component or a solid fuel, and such fuel is supplied in the first reaction zone. This provides an advantage that when a fuel portion of the material which has a fuel containing component has a low ignition temperature and during the combustion in an oxygen rich stream of hot gas, it can give off its heat through the non-combustible material components and the treatment material.
In a preferred arrangement, the first reaction zone has a temperature range such that the easily volatile constituents of the fuel containing material/or the solid fuel burn out completely whereas the constituents which volatilize with difficulty and the solid constituents will burn out only partially. Thus, the still unburned solid fuel constituents then are included with the stream of comminuted raw material and separated out from the gases and passed into the second reaction zone. In the second reaction zone, a separate fuel is added such as gas or oil and a higher combustion temperature occurs. The solid fuel constituents which were not burned out in the first reaction zone are intensively mixed with the material and some may be additionally burned out during the separation which occurs between the first and second reaction zone, but the solid fuels will additionally complete their combustion in the second reaction zone where they are fired with the gas or oil, and thus, an optimal calcination treatment results. It is possible even with high output yields and with a high material to gas relation to achieve an optimal accumulation or mixture between the fuel particles and the material particles and to attain improved heat transfer required for the calcination.
In accordance with a feature of the invention, the second reaction zone is supplied directly with an oxygen containing gas, such as from the sintering step. Thus, the sintering step exhaust gases which still contain oxygen are utilized for the calcination with the additionally supplied fuels in this reaction zone. It is for this reason that the first reaction zone is connected in series with separately supplied combustion air. By the division of the reaction steps of calcination into two reaction zones, the first reaction zone provides an almost complete calcination treatment of the material through combustion of a lower grade fuel in an oxygen rich atmosphere while in the next following reaction zone, the remaining calcination is carried out by means of a higher grade fuel in the furnace exhaust atmosphere enriched with oxygen and some mineral clinker formation may already be produced at that step. It is possible that even with the greatest output yields, the calcination step of the process may be essentially completed so that the mechanical installation of the kiln may be held optimally small. It is also suitable that the combustion air supplied in the first reaction zone is hot exhaust air from the cooling step which provides a good thermal effiency for the entire installation.
The invention relates to a device for the thermal treatment of pulverulent material which ranges from fine grain to dust size with hot gases, particularly for the calcination of cement including a suspension gas heat exchanger which is arranged to be connected in series with a rotary kiln and a grate cooler and has two partial calcination sections for the precalcination of the material which are connected consecutively and each partial calcination section has a separate fuel feed. The advantages of this arrangement include that without additional expenditure for substantial structure, the period of dwell of the treatment material is appreciably prolonged so that even in cement installations of high output yields, sufficient period of dwell in a hot gas atmosphere is available without exceeding the limit of temperatures for the material. The invention provides that between partial calcination sections of a suspension gas heat exchanger of the cylone type, an insertion structure may be provided for conveyance of the material into partial calcination sections connected in series so that an intensive mixture of at least partially calcined material may take place before proceeding into the next following reaction section.
It is a further feature of the device that partial calcination sections are provided connected in series with the first section being provided with a separate combustion air supply, preferably in the form of an air conduit leading from the clinker cooler exhaust air. This provides the lowest technical or structural installation expenditure so that by means of the supply of a high grade fuel into an oxygen rich combustion atmosphere, the material may be calcined up to 90% to 95% almost completely. In the second section connected in series by the supply of a correspondingly measured residual quantity of fuel, the calcination treatment of the material may be carried out to 100%, and if desired, mineral clinker formation of the C.sub.2 S phase may be introduced. The two step calcination combustion section is then completely integrated into a suspension gas heat exchanger, and it is possible to set up a cement production installation with a so-called short kiln even for the largest output yield so that investment costs are lowered, and the total economy of the cement manufacturing installation is improved.
Other objects, advantages and features, as well as equivalent structures and methods which are intended to be covered herein, will be more apparent from the teaching of the principles of the present invention in connection with the disclosure of the specification, claims and drawings, in which: